An intrinsic (i-type) microcrystalline silicon thin film has been widely used as a photoelectric conversion layer of a silicon thin film solar cell. As a method of manufacturing a microcrystalline silicon thin film, it is general to perform deposition on a substrate by a plasma-enhanced chemical vapor deposition (CVD) method using a mixed gas of silane (SiH4) and hydrogen (H2).
In general, a film forming method called a “high-pressure depletion method” has been widely used to deposit the microcrystalline silicon thin film through the plasma CVD method (for example, see Patent Document 1). Specifically, in a high pressure atmosphere, a SiH4 gas flow rate [SiH4] is sufficiently reduced (that is, a H2 gas flow rate [H2] is sufficiently increased), the flow rate ratio of SiH4 “[SiH4]/([SiH4]+[H2])” is reduced to about 1% to 5%, and SiH4 in plasma is depleted. In this way, it is possible to deposit the microcrystalline silicon thin film (conversely, when the flow rate ratio of SiH4 is high, an amorphous film is deposited). The microcrystalline silicon thin film thus obtained by this method is applied to a photoelectric conversion layer of a solar cell to manufacture a solar cell sample, and the solar cell sample is evaluated. As a result, practical characteristics, such as a photoelectric conversion efficiency of up to about 9%, are obtained.
As another method of forming the microcrystalline silicon thin film, a film forming method called a layer-by-layer method has been known (for example, see Non-patent Literature 1). In the layer-by-layer method, a plasma CVD apparatus is used to alternately repeat a process of generating SiH4 plasma in the range of 10 s to 100 s and a process of generating H2 plasma in the range of 100 s to 600 s plural times to thereby deposit a silicon thin film. Then, the thin film, which is an amorphous state when it is deposited, is continuously exposed to H2 plasma such that it is crystallized.
In recent years, a large substrate with a size of 1 m×1 m or more has been used in order to put a silicon thin film solar cell to practical use and manufacture a solar cell with high productivity and at a low cost. In the formation of the microcrystalline silicon thin film, not only the deposition rate and the quality of a film, but also a large-scale uniform film forming technique for uniformly forming a thin film on a large meter (m)-size substrate become important.